The conventional recording media comprises two types: memory and disk. Functionally, the memory could be divided into three kinds of silicon-base memory such as read only memory (ROM), flash memory and read access memory (RAM).
The structure of the memory is quite simple, which is constructed by lots of tiny transistors that could be recharged repeatedly. Each tiny transistor is in a charged or non-charged status. When the transistor is charged or non-charged, the status of the transistor will be represented by “1” or “0” so as to record the data.
Except the aforesaid three kinds of memory, there still are memories comprising new recording layers, such as ferroelectric memory (FRAM), phase change memory (PRAM), magnetoresistive random access memory (MRAM) or resistance memory (RAM) . . . , etc.
FRAM uses the residual polarized properties of the ferroelectric materials to control the field-effect conductivity of the semiconductor to represent the memory properties. When an electric field is applied to the ferroelectric crystal, the central atoms are moved with the direction of the electric field. Due to the motion of the atoms is inside the crystal, a charge spike is generated by passing an energy barrier. An inside-circuit could sense the charge spike and processes the memorization. When the electric field is removed from the crystal, the central atoms will remain at a proper position to keep the memorization status.
PRAM is a non-volatile memory, which is the same with the common flash memory, but the principle of the data reading or writing of these two kinds of memories are different. The operating principle of the PRAM is very similar with an optical disk, which uses a so-called sulfur compounds such as germanium (Ge), antimony (Sb), tellurium (Te) as a core, and changes the material status between crystalline and amorphous by heating under the control of heating parameters and dissipating conditions, so as to generates different resistance to record the signals such as 0 or 1 by changing the crystalline status of the materials.
The inside structure of the MRAM is composed by one transistor and a magnetic tunnel junction storage unit (MTJ). The MTJ has three layers therein, the top layer is a free layer, the middle layer is a tunnel junction, and the bottom layer is a fixing layer. The polarized direction of the magnetic field of the free layer is fixed. When the directions of the magnetic field of the free layer and the fixing layer are parallel, the storing unit is low resistive; and when the directions are opposite, the storing unit is high resistive. By detecting the resistance of the storing unit, the MRAM could determine that the data recorded is 0 or 1.
The operating principle of the RRAM is to apply an external voltage at two ends of the electrodes, and so change the resistance of the metal oxide from high to low. By using these two kinds of resistance configuration, the function of the memory could be completed, which means that at high resistance status is 1 and at low resistance status is 0.
In the disks, except the CD-ROM which the data is pre-cast on the substrate needs no recording layer, the rest disks all need the recording layer to achieve the function of recording data on the disk. The recording layers are substantially categorized two different kinds of one-time recoding layer and rewriting recording layer.
For the write-once recoding layer, the disk is changed by emitting light thereon, and the disk change will cause the change of the light reflection. The data 1 or 0 is generated by determining the strength of the reflected laser. There are two kinds of recording way of write-once recoding layer: hole-burning type and phase change type. For the hole-burning type recoding layer, the part of the recording layer emitted by the laser is melted and a hole is formed at the emitted site, and it causes the interference of the laser and changes the intensity of the reflected laser. On the other hand, for the phase change type recoding layer, the reflectance of the recording layer itself is changed by applying a high intensity laser to the recording layer and so to change the crystalline status of the recording layer.
For the rewritable recording layer, there are two types of way to achieve the goal: magnetic-optical recording and phase change type. The writing process of the former is to focus the laser on the recording layer, and heat the emitted region to near the Curie temperature by the ferromagnetism recording material absorbing the heat. The inside magnetic molecular are thus at an unstable arranged state, and the magnetic structure of the heated part is becoming disordered and the coercive force is becoming zero, thus the magnetic moment is easy to change with an applied external magnetic field. By applying a vertical magnetic field near the heated region, the magnetization direction of the magnetic molecular is aligned upward or downward. When removing the laser and thus decreasing the temperature of the recording layer, the coercive force will back to the original status, so the magnetic moment of this region is remained along the new direction, and the arrangement of the magnetic molecular is fixed, which the upward represents 1 and downward represents 0. The erasing is the same as writing, which heats the disk material to higher than the Curie temperature, and applies a fixed downward magnetic field so as to write the disk with 0.
The method of reading the data is to use the magneto-optical Kerr effect. When reading the data recorded on the disk, a linear polarized laser is incident to the recording layer of the magneto-optical disk. When the reflected light is reflected from the surface, the incident light is effected by the vertical magnetization (M) of the magneto-optical material, and it generates a relative rotation angle (θk) called Kerr angle between the reflected light or transmitted light and the original incident polarized light. Assume that the Kerr angle of the upward magnetization direction vertical to the thin film is +θk, then the Kerr angle of the downward magnetization direction will be −θk. By representing the +θk and the −θk with 0 and 1 respectively, the Kerr angle of the reflected laser could be determined positive or negative, and the serial digital signals pre-recorded on the magneto-optical disk could be read. Thus users could repeat the operations of writing, reading and erasing until the material fails.
For the phase change disk, through rapidly heating and cooling the disk by a laser, the crystalline status of the disk having a 30% reflectance is change to amorphous state and the disk having a 15% reflectance. Different to the read-only disk, the rewritable disk could recover to the original crystalline status by slowly increasing and decreasing the temperature, so as to achieve the goal of re-writing.
However, due to the resolution limitation of the reflected light, only two-level recording (0 and 1) could be processed, and this limits the development of a huge-capacity storage apparatus such as a memory. Therefore, a new recording method is required to raise the data recorded amount of single recording spot of the memory or the storage apparatus.